Gpio: STM32: remove setAlternateFunction and clean up code.

setAlternateFunction is still available as a private
method, but it behaves differently: It does not change
OutputType, OutputSpeed or PullType anymore.
The way to use Gpio in the next XPCC release is going to be:
GpioA0::setOutput();
GpioA0::connect(Timer1::Id); // uses setAlternateFunction internally